Systems and methods for analyzing stability using metal resistance variations

ABSTRACT

This disclosure describes techniques for analyzing statistical quality of bitstrings produced by a physical unclonable function (PUF). The PUF leverages resistance variations in the power grid wires of an integrated circuit. Temperature and voltage stability of the bitstrings are analyzed. The disclosure also describes converting a voltage drop into a digital code, wherein the conversion is resilient to simple and differential side-channel attacks.

This application is a continuation of U.S. National application Ser. No.14/907,423, which claims the benefit of International ApplicationPCT/US2014/053279 filed Aug. 28, 2014, which claims the benefit of U.S.Provisional Application No. 61/870,974, filed on Aug. 28, 2013, all ofwhich are incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under CNS1018748 awardedby National Science Foundation (NSF). The government has certain rightsin the invention.

TECHNICAL FIELD

This disclosure relates to systems and methods for generating randombitstrings and more particularly, to Physical Unclonable Functions(PUFs).

BACKGROUND

Random bitstrings may form the basis for encryption, identification,authentication, and feature activation in hardware security. In currenttechnologies, keying material for encryption may be stored as digitalbitstrings in non-volatile memory on field-programmable gate arrays(FPGAs) and application-specific integrated circuit (ASICs). However,secrets stored this way may not be secure against a determinedadversary, who can use probing attacks to steal the secret. PhysicalUnclonable Functions (PUFs) may be used as an alternative to storingdigital bitstrings in non-volatile memory. PUFs may leverage randommanufacturing variations in integrated circuits as the source of entropyfor generating random bitstrings, and may incorporate an on-chipinfrastructure for measuring and digitizing the correspondingvariations.

The quality of a PUF may be judged based on one or more of uniquenessamong a population, randomness of the bitstrings produced, andreproducibility or stability across varying environmental conditions(i.e., temperature and voltage). The quality of current PUFs may be lessthan ideal. Further, current techniques for determining the uniqueness,the randomness, and the stability of PUFs may be less than ideal.

SUMMARY

In general, this disclosure describes techniques for generating aphysical unclonable function (PUF). In particular, this disclosuredescribes techniques for producing a PUF based on resistance variations.This disclosure describes analyzing statistical qualities of bitstringsproduced by a PUF of a circuit. Specifically, a PUF that leveragesresistance variations in the polysilicon and metal wires of the circuitis analyzed at different temperatures and voltages to determine itsstability. The disclosure also describes converting a voltage drop of acircuit into a digital code, wherein the conversion is resilient tosimple and differential side-channel attacks.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example of an integratedcircuit architecture that may implement one or more techniques of thisdisclosure.

FIG. 2 is a schematic diagram illustrating an example of aStimulus/Measure circuit (SMC) that may implement one or more techniquesof this disclosure.

FIGS. 3A-3B are graphs illustrating example power grid voltagedistributions for an example integrated circuit implementing one or moretechniques of this disclosure.

FIG. 4A-4B are graphs illustrating of an example bit generation processusing an example PUF described herein.

FIG. 5 is a graph illustrating an example distribution of hammingdistances for example bitstrings generated according to techniques ofthis disclosure.

FIG. 6 is a graph illustrating the pass-fail rate of example integratedcircuits implementing one or more techniques of this disclosure.

FIG. 7A is a conceptual diagram illustrating an example bitstringredundancy technique of this disclosure.

FIG. 7B is a conceptual diagram illustrating an example bitstringredundancy technique of this disclosure.

FIG. 8 is a graph illustrating an example technique for improvingbitstring stability.

FIG. 9 is a schematic diagram illustrating an example of avoltage-to-digital converter (VDC) that may implement one or moretechniques of this disclosure.

FIG. 10 is a graph illustrating an example distribution of hammingdistances for example bitstrings generated according to techniques ofthis disclosure.

FIG. 11 is a schematic diagram illustrating an example of avoltage-to-digital converter (VDC) that may implement one or moretechniques of this disclosure.

FIG. 12 is a schematic diagram illustrating an example of avoltage-to-digital converter (VDC) that may implement one or moretechniques of this disclosure.

FIG. 13A-13B are graphs illustrating of an examples measuring differencevalues for use in an example PUF described herein.

FIG. 14 is a schematic diagram illustrating an example of a circuit thatmay implement one or more techniques of this disclosure.

DETAILED DESCRIPTION

Physical Unclonable Functions (PUFs) are promising components for nextgeneration integrated circuit (IC) security. PUFs derive random, butreproducible, bitstrings that can be used in security applications suchas encryption, authentication, feature activation, metering, etc.Bitstrings may be generated on-the-fly using dedicated hardwareprimitives and processing engines, and thereby may avoid the need forstorage in on-chip non-volatile memories. This feature may not onlyimprove their resilience to invasive attacks designed to steal secretkeying material, but may also reduce the cost of manufacturing an IC.That is, in many cases, PUFs may be designed using components that canbe fabricated using standard CMOS processing steps, and therefore, thecost of integrating non-standard components, such as non-volatilememories, may be eliminated. Another important characteristic of PUFs asa next generation security mechanism is their potential for generatinglarge numbers of repeatable random bits. This feature offers newopportunities for software processes to strengthen security mechanisms,for example, by allowing frequent rekeying in encrypted communicationchannels and by allowing a large, changing set of shared keys to beutilized among multiple communicating entities.

PUFs are designed to be sensitive to variations in the printed andimplanted features of wires and transistors on the IC. Precise controlover the fabrication of IC components is becoming more difficult inadvanced technology generations, resulting in a wider range ofelectrical variations among and within the replicated copies of a chip.Signal variations that occur within the IC may be the source of entropyfor the PUF. Example PUF implementations may leverage variations intransistor threshold voltages, in speckle patterns, in delay chains andring oscillators, in thin-film transistors, in SRAMs, in leakagecurrent, in metal resistance, in optics and phase change, in sensors, inswitching variations, in sub-threshold design, in read only memories(ROMs), in buskeepers, in microprocessors, using lithography effects,and aging. It should be noted that PUFs may incorporate different typesof on-chip infrastructures for measuring and digitizing the same type ofvariation. Although, in some examples, the techniques described hereinare described with respect to PUFs based on metal resistance variations,the techniques described herein may be generally applicable to any typeof PUF.

Several statistical criteria have emerged as important metrics forjudging the quality of a PUF. Interchip hamming distance (HD) may beused to determine the uniqueness of bitstrings among a population ofchips. Similarly, the National Institute of Standards and Technology(NIST) statistical test suite can be used to evaluate the randomness ofthe bitstrings produced by each chip. Intra-chip hamming distance (HD)may be used to evaluate stability of bitstrings. That is, the ability ofeach chip to reproduce the same bitstring time-after-time, under varyingtemperature and voltage conditions.

The ability of each chip to reproduce the same bitstring time-after-timemay be described with respect to the ability of each chip to avoid bitflips, where a bit-flip may be defined as ‘0-to-1’ and ‘1-to-0’ changein a generated bitstring as temperature and voltage are varied. Thisdisclosure describes an example PUF and evaluates its stability.Further, this disclosure describes an example bit-flip avoidance schemethat reduces the probability of a failure to reproduce the bitstring.The example bit-flip avoidance scheme was evaluated and shown to reducethe probability of a failure to reproduce the bitstring to less than1E-9.

As described above, PUFs may incorporate different types of on-chipinfrastructures for measuring and digitizing the same type of variation.Different types of on-chip infrastructures may provide differentstabilities for a particular PUF. This disclosure describes exampleon-chip voltage-to-digital converters (VDC) for measuring voltagevariations. In one example, voltage variations may reflect resistancevariations in the metal wires. The stability of the example on-chipvoltage-to-digital converter was evaluated at different temperatures andvoltages.

FIG. 1 is a block diagram illustrating an example of an integratedcircuit architecture that may implement one or more techniques of thisdisclosure. In the example illustrated in FIG. 1, chip 100 is based on a90 nm chip architecture. In other examples, chip 100 may be based onother architectures (e.g., 65 nm, architectures 45, nm architectures,etc.) In one example, the 90 nm chip architecture may be fabricatedusing IBM's 90 nm, 9 metal layer bulk silicon process. As illustrated inFIG. 1, the padframe of chip 100 includes 56 I/Os, and surrounds a chiparea of approx. 1.5 mm×1.5 mm. Two PADs labeled PS (Psense) and NS(Nsense) along the top of chip 100 refer to voltage sense connections,where PS may be used for sensing voltages near V_(DD) and NS may be usedfor sensing voltages near GND. In the example illustrated in FIG. 1,each of PS and NS terminals wire onto chip 100 and connect to 85 copiesof a Stimulus/Measure circuit (SMC). In the example illustrated in FIG.1, the SMCs are illustrated as small rectangles and are distributedacross entire chip 100 as two arrays, a 7×7 outer array and a 6×6 innerarray (49 SMCs in outer array and 36 SMCs in inner array for a total of85 SMCs). Although not shown in FIG. 1, a scan chain may connectserially to each of the SMCs to allow each of them to be controlled.

FIG. 2 is a schematic diagram illustrating an example of aStimulus/Measure circuit (SMC) that may implement one or more techniquesof this disclosure. In the example illustrated in FIG. 2, SMC 200includes a pair of shorting transistors 3 a and 3 b. In one example,each of the shorting transistors may be capable of sinking approximately10 mA of current through the power grid when enabled. In this example,the resulting voltage drop/rise on the V_(DD) and GND, respectively maybe less than 10 mV. As illustrated in FIG. 2, SMC 200 includes a set of16 “pseudo” transmission gates (TGs), labeled 1 a through 1 h, in FIG.2, that serve as voltage sense devices. As illustrated in FIG. 2, eightof the TGs 1 a-1 h connect to eight of the nine metal layers (the ninthmetal layer is not shown in FIG. 2) that define the V_(DD) stack-up ofthe power grid, as shown on the left side of FIG. 2 (labeled M₁ throughM₈), while the other eight TGs 1 a-1 h connect to the GND stack-up, asshown on the right side of FIG. 2 (labeled M₁ through M₈).

In SMC 200, scan FFs 4 a and 4 b and 3-to-8 decoders 5 a and 5 b allowexactly one of the TGs to be enabled in each of the stack-ups. Asillustrated in FIG. 2, SMC 200 includes additional TGs 2 a and 2 b thatconnect to the drains of the each of the 8 stack-up TGs, one for V_(DD)and one for GND. Separate scan FFs control their connection to thechip-wide wires that route to the PS and NS pins illustrated in FIG. 1.The configuration and control mechanism of SMC 200 allows any V_(DD) andGND voltage to be measured using off-chip voltmeters.

In one example, a challenge may be applied to circuit 100 by configuringthe scan chain to (1) enable the shorting transistors within an SMC, and(2) enable two TGs in that same SMC. For example, with respect to SMC200, shorting transistors 3 a and 3 b may be enabled and the TG labeled2 a and one TG from the group 1 a through 1 h may be enabled. Once thetransistors are enabled, a voltage drop/rise may be measured on the NSand PS pads using voltmeters. A voltmeter may include an off-chipvoltmeter or an on-chip voltmeter.

In one example, in order to reduce bias effects and correlations thatexist in the V_(DD) and GND stack-ups, inter-layer voltage drops/risesmay be computed by subtracting pair-wise, the voltages measured fromconsecutive metal layers, i.e., V_(M1)-V_(M2), V_(M2)-V_(M3), etc. Thesevoltage differences, which may be referred to as power grid voltagedifferences (PGVDs), may also allow the PUF to leverage the independentresistance variations that occur in each of the metal layers of thepower grid. It should be noted that a significant benefit of using metalstructures based PUF is that “noise-related” variations, such as thoseintroduced by temperature and voltage (TV) variations, result in linearchanges to the measured voltages. This linear scaling characteristicallows the relative magnitude of two voltages to remain consistentacross changes in temperature and voltage, which, in turn, improves thestability of the PUF to bit-flips, when compared, for example to PUFswhich leverage transistor-based variations.

The eight TGs in the respective V_(DD) and GND stacks as shown in FIG. 2indicate that 7 PGVDs can be computed per stack. However, it should benoted that the structure of the power grid on the chips may reduce thevoltage drops on the upper layers of the power grid. Therefore, in somecases analysis can be restricted to PGVDs generated using the lower fourmetal layers (i.e., M₁ through M₄), which allows three PGVDs to becomputed. Therefore, in one example, each chip 100 in a population maygenerate 85 SMCs*3 metal layer pairings=255 PGVDs for each of the V_(DD)and GND stacks. Each of the PGVDs can be compared with other PGVDs invarious combinations to produce a bitstring. In one example, analysismay be conducted on bitstrings generated by comparing each PGVD with allothers generated using the same metal layer pairing. Therefore, in oneexample, the total number of bits per chip 100 may be equal to 85SMCs*84/2 per metal layer pairing*3 metal layer pairings*2grids=3,570*6=21,420 bits. Thus, in one example, circuit 100 may include85 of SMC 200 and may be configured to implement a PUF capable ofproducing 21,420 bits. This PUF may be referred to in this disclosuregenerally as a power grid (PG) PUF and more particularly as PGPUF₁.

As described above, the quality of a PUF may be determined based on oneor more of uniqueness, randomness, and stability. In one set ofexperiments, PGPUF₁ was evaluated at nine TV corners, i.e., over allcombinations of three temperatures; negative 40° C., 25° C. and 85° C.,and three voltages; nominal and +/−10% of nominal. The stability of thebitstrings produced using PGPUF₁ was measured using intra-chip HD and‘probability of failure’ techniques. Further, the randomness anduniqueness of bitstrings produced using PGPUF₁ were also evaluated usingthe NIST test suite and inter-chip HD methods. It should be noted thatthe order in which the comparisons were made was randomized. In anon-chip implementation, this can be accomplished using a linear feedbackshift register (LFSR) and a seed. In the experiments, digitized voltageswere obtained from an off-chip voltmeter. The process of randomizingcomparisons was modeled in experiments using the functions srand(seed)and rand( ) from the C programming library. Further, the randomness,uniqueness and stability characteristics of PGPUF₁ was evaluated for aset of 63 chips.

Based on the experiments it was found that unstable bits, defined asbits that are susceptible to flipping because their PGVDs are verysimilar, actually reduce several quality metrics associated with theoverall bitstring, including inter-chip HD and NIST statistical testscores. Moreover, including unstable bits in a bitstring may require theinclusion of error correction and/or Helper Data schemes. Errorcorrection and Helper Data schemes may weaken security and increaseoverhead. This disclosure describes an example bit-flip avoidance schemethat may be used to identify and discard unstable bits. The examplebit-flip avoidance scheme may be used as an alternative to errorcorrection and Helper Data schemes. The example bit-flip avoidancescheme may be referred to as thresholding.

Thresholding may be carried out by first computing a threshold from thedistribution characteristics of the PGVDs. Computing a threshold fromthe distribution characteristics of the PGVD is illustrated in FIGS.3A-3B. FIGS. 3A-3B are graphs illustrating example power grid voltagedistributions for an integrated circuit. FIGS. 3A-3B illustrate the GNDand V_(DD) PGVD distributions for a sample chip, CHIP₁. Eachdistribution in FIG. 3A and FIG. 3B includes 255 PGVD values, derived asdescribed above. That is, each chip 100 in a population may generate 85SMCs*3 metal layer pairings=255 PGVDs for each of the V_(DD) and GNDstacks.

In the examples illustrated in FIGS. 3A-3B, the distance between the 10%and 90% points in the distributions is used to derive the thresholds forthe thresholding algorithm. As illustrated in FIGS. 3A-3B, the distancebetween the 10% and 90% points are approximately 0.3 mV for GND PGVDsand 0.15 mV for the V_(DD) PGVDs for this chip. The limits at 10% and90% are used to avoid distortions caused by potential outliers in thePGVD values for each chip. In other examples, different percentages maybe used to derive the thresholds (e.g., 5% and 95%, 15% and 85%, 20% and80%).

The thresholds may then be scaled by a constant to produce the actualthreshold used during bit generation. FIG. 4A and FIG. 4B provide anillustration of the bit generation process using the GND PGVDs forCHIP′. Both the graph illustrated in FIG. 4A and the graph illustratedin FIG. 4B plot the bit comparison number along the x-axis against thevalue of the difference between the two PGVDs being compared. Only thebits that survive the thresholding are included in the plots, i.e., thex-axis shows only about half of the 10,710 comparisons. The bits thatsurvive the thresholding may be referred to as strong bits. Points thatappear in the upper portion of the FIGS. 4A-4B generate a ‘1’ bit whilepoints in the lower portion generate a ‘0’ bit.

FIG. 4A shows only the points obtained from enrollment, which wascarried out at 25° C., 1.2 V. The thresholds are depicted using twohorizontal lines at 0.11 and −0.11 mV. These values were obtained byscaling the 0.3 mV obtained from the distribution (i.e., 10% and 90%points) by a constant of 0.37 (i.e., 0.3 mV*0.37=0.11 mV). It should benoted that in this example, the scalars 0.37 and 0.65 for the GND andV_(DD) thresholds, respectively were determined to be sufficient toprevent bit flips in chips. In other examples, other scalars may beused. FIG. 4B adds in the data points from the remaining 8 TV(regeneration) corners. Close inspection of the graphs in FIGS. 4A-4Breveals that some of the data points from regeneration appear within thethreshold band of width 0.22 mV, centered around 0.0 mV. Noise thatoccurs during regeneration causes points to move vertically, but as longas no points move across the 0.0 line, no bit-flips occur.

The usage scenario that enables this process to be applied in situationswhere exact regeneration of a bitstring is required works as follows.During the initial bitstring generation, thresholding is used toidentify the unstable bits. For each unstable bit, its numbered positionin the sequence of challenges applied to generate the bitstring isrecorded in public storage. Later, during regeneration, thresholding isdisabled and public memory is consulted to determine which challenges toapply during bit generation.

The results of applying the thresholding technique to 63 chips testedunder nine TV corners are described below with respect to FIG. 5 andFIG. 6. An important concern regarding the thresholding technique dealswith the fraction of bits that survive it. In experiments that wereconducted, it was found that this fraction was different for the GND andV_(DD) stacks. On average, approximately 50% of the comparisons usingthe GND PGVDs survive the thresholding, while only 22.5% of thecomparisons survive using the V_(DD) PGVDs. The lower value for theV_(DD) PGVD analysis is believed to occur because of the increased noiselevels on the V_(DD) grid, relative to the GND grid. As a consequence,the average bitstring length reduces to approximately 7,765 bits(36.25%) from the original size of 21,420 bits. These bitstrings werefound to be reproducible at all of the nine TV corners.

The true average intra-chip HD, which is a measure of the underlying bitstability across the TV corners, is computed as 4.01%. This value isobtained by analyzing the full length, i.e., 21,420-bit, bitstrings withthresholding disabled and counting the number of times a bit-flip occursin each bit position across all pairings (9*8/2=36) of the bitstringsproduced under each of the nine TV corners for each chip. The averageintra-chip HD, expressed as a percentage, is obtained by dividing thenumber of bit flips by 36*21,420, which is the total number of bitpairings inspected for each chip, and multiplying by 100. The valuereported is the average of these percentages across all chips. Any valueless than 5% is considered high quality according to the publishedliterature on PUFs.

Interchip HD, as described above, measures the uniqueness of thebitstrings, where the best possible result is 50%, i.e., on average,half of the bits in the bitstrings of any two arbitrary chips aredifferent. FIG. 5 is a graph illustrating an example distribution ofhamming distances for example bitstrings generated according totechniques of this disclosure. FIG. 5 plots the distribution ofinterchip HDs. The 1,953 HDs included in the distribution of FIG. 5 areobtained by pairing the stable bitstrings from all chips under allcombinations. The chip with the shortest stable bitstring is used to setthe size of the bitstrings used in each HD calculation, requiring allbitstrings to be truncated to 7,343 bits. The average HD is 3,666.8(49.94%), which is close to the ideal HD of 3761.5 (50%).

Experiments also evaluated randomness using the NIST statistical testsat the default significance level of 0.01. Given the relatively shortlength of the stable bitstrings, only 11 of the 15 NIST statisticaltests are applicable. FIG. 6 is a graph illustrating the pass-fail rateof example integrated circuits implementing one or more techniques ofthis disclosure. The bar graph shown in FIG. 6 gives the number ofpassing chips on the z-axis for each of the 10 tests on the x-axis, andfor each of 10 different seeds on the y-axis. The number of passingchips is in reference to passing the null hypothesis. The nullhypothesis is specified as the condition in which thebitstring-under-test is random. Therefore, a good result is obtainedwhen the number of chips that pass the null hypothesis is large.

With 63 chips, NIST requires that at least 60 chips produce a p valuethat is larger than the significance level (α=0.01), otherwise the wholetest is considered ‘failed.’ In the graph illustrated in FIG. 6,overall, of the 11*10=110 bars, 41 are full height indicating that all63 chips passed the test, 39 bars have height 62, 27 have height 61 and2 have height 60. Therefore, 109 bars of the 110 are equal to or largerthan the required value to pass the test, and only one bar is below thethreshold at 58. The bars for the Non-Overlapping Template testsrepresent the average pass rate across all 148 individual tests. Of the10*148=1,480 individual tests, 61 failed, 59 tests by no more than twochips, one by three chips (57 chips passed) and one by four chips (56chips passed). Moreover, all but eight of Pvalue-of-the-Pvalues testspassed, indicating the P-values are uniformly distributed between 0.0and 1.0. The fails in this category occurred in the Rank andNon-Overlapping Template tests, both which NIST recommends testing withmuch larger bitstrings than those used here. Overall, these are verygood results and indicate the bitstrings generated using the PG PUF arecryptographic quality.

The large size of the bitstrings produced by the PUF can be used tofurther enhance their reliability over that provided by thresholdingalone. This can be accomplished by creating three copies of afixed-length bitstring from the sequence of strong bits produced by thePUF. The three copies can then be compared as a means of avoiding bitflips, in the spirit of a popular scheme used in fault tolerance calledtriple-module-redundancy or TMR. TMR is based on a majority votingscheme in which the final bit for a given bit position is obtained bytaking the majority across all three copies of the bitstrings.

This technique was investigated using fixed-length bitstrings of256-bits. FIG. 7A is a conceptual diagram illustrating an examplebitstring redundancy technique. In one example, a TMR-based bitstring iscreated during enrollment by copying the first 256 strong bits into thefirst copy of the fixed-length bitstring as illustrated in FIG. 7A. Thesecond two copies are created by parsing the remaining strong bits,searching for matches to the first copy. It should be noted that theTMR-based bitstring of length n requires approximately 5*n strong bitsto construct. As described above for thresholding, the positions of thematching bits are indicated by writing a ‘1’ in the public storagebitstring (not shown), while the positions of the skipped bits (and theweak bits encountered under thresholding) are indicated by writing a‘0’. Later, during regeneration, the public storage bitstring isconsulted to determine which challenges are to be used to reconstructthe three copies of the bitstring. Once created, the final bitstring isobtained by majority vote on each column as shown in FIG. 7A. Thisallows the correct bitstring to be generated despite any singlebit-flips that may occur in a column, such as the one shown in FIG. 7Aat the last column of Redundant BS₁.

FIG. 7B illustrates a proposed thresholding and TMR-based scheme usingdata from a hypothetical chip. The x-axis in FIG. 7B plots a sequence ofcomparisons that may be used to generate a bitstring, while the y-axisplots the differences between the pairings of PGVDs. Each differencereflects the relative ordering of the two PGVDs, e.g., positivedifference values indicate that the first PGVD is larger than thesecond. For strong bits, in one example, the PGVD difference data pointsmust lie above or below the thresholds, labeled ‘+Tr’ and ‘−Tr’ in FIG.7B. This condition, when met, is recorded using a ‘1’ in thethresholding bitstring shown below the data points. Weak bits, on theother hand fall within the thresholds and are indicated with a ‘0’. Weakbits, which are denoted as underlined ‘0’s in FIG. 7B indicate strongbits that are skipped under the TMR scheme described below. The exampleTMR-based technique illustrated in FIG. 7B constructs 3 identicalbitstrings during enrollment, labeled ‘Secret BS’, ‘Redundant BS1’ and‘Redundant BS2’ as shown along the bottom of FIG. 7B. The first strongbit encountered in the left-to-right sequence is placed into the firstbit position of the ‘Secret BS’ copy. This ‘1’ bit constrains the firstbit of the redundant BSx to ‘1’. Therefore, the left-most strong ‘0’ bitencountered next needs to be skipped. The two redundant strong ‘1’s arefound in column positions 5 and 6. The next strong bit, a ‘0’ in thiscase, defines the second bit in the 3 bitstrings and the processcontinues. The number of strong bits required to generate a secretbitstring of length 4 is approx 5× or 20. From the example, this isevaluated by counting the number of ‘1’s and bolded ‘0’s in thethresholding bitstring, which is given as 18.

In order to illustrate the improvement provided by TMR over voltagethresholding alone, the GND threshold scalar given above as 0.37, wasiteratively decreased in 0.01 steps down to 0.0. As the threshold wasdecreased, bit flips begin to occur in the thresholding-only bitstrings.A thresholding-only ‘probability of failure’ curve can be constructed bycounting the number of bit flips that occur in the bitstrings from all63 chips and dividing it by the total number of bits. A similar curvecan be constructed using TMR, but in this case, a bit flip is notcounted unless it occurs in two or more of the three bits of a column asshown in FIG. 7A. Moreover, the total number of bits used in thedenominator for the TMR-based curve is reduced by a factor of three toaccount for the actual number used in the final TMR-based bitstring.

FIG. 8 plots the data points for these two curves as well as two‘exponential-curve’ fits to them. In FIG. 8, the GND threshold scalingconstant is plotted along the x-axis against the probability of failureon the y-axis. The exponential curve fits allow the probability offailure to be predicted for thresholds beyond (i.e., to the right) ofthe last recorded bit flip in the relatively small population of chips.For example, the probability of failure using voltage thresholding aloneat the 0.37 threshold is 6.5E-7. This improves by nearly three orders ofmagnitude to 2.4E-10 using the TMR-based scheme. Of course, theTMR-based scheme can be expanded to further improve bit-flip resilienceby generating five (or more copies) of the bitstring, at the expense ofincreased usage of bits and public storage size.

In the experiments described above, digitized voltages were obtainedfrom an off-chip voltmeter. As described above, PUFs may be implementedusing on-chip voltage-to-digital converters. Each of FIG. 9, FIG. 11,and FIG. 12 illustrate examples of a voltage-to-digital converter (VDC)that may implement one or more techniques of this disclosure. Theexample on-chip VDC 900 illustrated in FIG. 9 is designed to ‘pulseshrink’ a negative input pulse as it propagates down an inverter chain.As the pulse moves down the inverter chain, it activates a correspondingset of latches to record the passage of the pulse, where activation isdefined as storing a ‘1.’ A thermometer code (TC), i.e., a sequence ofTs followed by a sequence of ‘0’s, represents the digitized voltage.

On chip VDC 900 works by introducing a fixed-width (constant) inputpulse, which is generated by the pulse generator 902 shown on the leftside of the FIG. 9. Two analog voltages, labeled Cal0 and Cal1 connectto a set of series-inserted NFET transistors in the inverter chain, withCal0 connecting to NFETs in even numbered inverters 904 a-904 n and Cal1to the NFETs in odd numbered inverters 904 a-904 n. An example of aninverter is illustrated in the call-out 906 on right side of FIG. 9. Thepropagation speed of the two edges associated with the pulse arecontrolled separately by these voltages. The pulse will eventually dieout at some point along the inverter chain when the trailing edge of thepulse ‘catches up’ to the leading edge. This may be ensured by fixingCal0 at a voltage higher than Cal1. The digital representation of theapplied Cal0 and Cal1 voltages can then be obtained by counting thenumber of sequentially stored ‘1’s in the latches 908 a-908 n.

As described above, PGVDs are created by subtracting the voltagesmeasured on consecutive metal layers in the power grid. Instead ofdigitizing these PGVs one-at-a-time with a VDC and then subtractingthem, the difference operation can be carried out in the analog domainby applying the two voltages from consecutive metal layers to the Cal0and Cal1 inputs. The larger PGV from the lower metal layer, M_(n), ofthe pair may be applied to Cal0 while the PGV from the adjacent, highermetal level layer, M_(n+1), may be applied to Cal1 (voltage drops areused for the V_(DD) grid voltages, e.g., V_(DD)−V_(Mn)).

FIG. 9 shows an example of how this may be accomplished. The PG array910 is configured to enable the PGV on M_(n) to drive the NS (or PS) pinand an off-chip voltmeter (VM) 912 is then used to digitize the value(same process as described for the experiments described above). The PGVis then multiplied by 15 and added to an offset, and the voltage sum isused to program an off-chip power supply 914 a which drives Cal0. Thesame process may be carried out for the PGV produced on metal layerM_(n+1) except the final value is used to program a second off-chippower supply 914 b which drives Cal1. In this example, themultiplication and offset operations are necessary because VDC 900requires the Cal0 and Cal1 voltages to be set between 500 mV and V_(DD)for proper operation.

It should be noted that unlike the off-chip voltmeter in the PGVexperiments in described above, the on-chip VDC 900 is subjected to thesame TV variations as the PUF (as would be the case in actualimplementation), and therefore its characteristics will vary as well. Inone example, a calibration process may be used to ‘tune’ the offsetvoltage to compensate for some of the changes in VDC 900 behavior, butsince the measurements are differential, VDC 900 is able toself-calibrate and cancel out most of the adverse effects of TVvariations by itself.

The same set of experiments were carried out and the same processes werefollowed as described above on the 63 chips using VDC 900 instead of anoff-chip voltmeter. The results were as follows. The average bitstringlength after thresholding was 8,388 bits (39.16%) and the shortest one(used to truncate the bitstrings from the other chips for thestatistical tests) was 7,506 bits. Both of these numbers are slightlylarger than the numbers obtained using the PGVs, as described in above,and indicates that the VDC 900 compensates for some of the TV variationsthat occur in the measured PGVs.

However, on the other hand, the statistical test results for the on-chipVDC-based bitstrings are slightly worse than those presented for thePGVs. FIG. 10 shows the HD distribution of the bitstrings and severalstatistical results, in the same graphical format as FIG. 5 for thePGVs. Although the interchip HD is close to the ideal of 50% at 49.87%,the value obtained for the PGVs is slightly better (49.94%). Moreover,the standard deviation of 46.3 bits given in FIG. 10 is slightly largerthan that given in FIG. 5 of 43.4 bits, primarily due to the longer tailon the left side of the graph in FIG. 10. The VDC-based bitstrings weretested using the same 11 NIST statistical tests listed in FIG. 6, butusing only the first seed. Although most tests were passed, the Runs andApprox. Entropy tests failed with 57 and 49 chips passing, respectively,and 20 of the 148 Non-Overlapping Template tests failed.

In summary, the digitization process carried out by on-chip VDC 900works well, but may not be as efficient as the off-chip voltmeters atremoving the bias that exists in the PGVs. In J. Ju et al., “Bit StringAnalysis of Physical Unclonable Functions based on Resistance Variationsin Metal and Transistors,” HOST, 2012, pp 13-20, which is incorporatedby reference in its entirety, it is shown that a ‘bowl-shaped’ patternexists in the M1 voltages across the 2-D array of SMCs and indicatedthat computing inter-metal layer voltage differences (as is done here)effectively eliminates it. A problem with using VDC 900 to compute theanalog difference directly deals with the different sensitivities thatexist for Cal0 and Cal1. In particular, Cal1 has higher sensitivity thanCal0, and therefore, the amplification factors for voltages applied toCal0 and Cal1 need to be different (as described above, 15 was used forboth factors in the experiments).

The asymmetry in the sensitivities behaves as follows. Assuming that theM_(n) voltage from VDC 900 increases by a fixed constant ΔV and theM_(n+1) voltage remains constant. Under these conditions, assuming theTC for these two measurements is equal to x. In contrast, a similarscenario where the voltage M_(n) remains fixed and the M_(n+1) voltageincreases by the same fixed constant ΔV does not result in the same TC.Instead, the TC is equal to y, where y>x. In other words, a delta changein the upper metal layer (M_(n+1)) voltage has a larger impact on thechange in the TC than it does for an equivalent lower metal layer(M_(n)) voltage change. Therefore, the TCs weigh the voltage change inthe lower metal layer less than a change in the upper metal layer, whichdistorts their relationship to the actual voltage difference.

A second problem with VDC 900 as shown in FIG. 9 is that it may besusceptibility to differential power attacks (DPA). A DPA is astatistical technique that is used to steal secrets embedded within ICs.It works by deducing internal states (and secrets) of an IC by analyzingpower supply transients that are generated from operating a functionalunit, such as the Advanced Encryption Engine. The pulse-shrinkingbehavior of VDC 900 makes it relatively easy to determine the TC codefor a given voltage difference measurement. The power transientgenerated by VDC 900 simply stops when the pulse shrinks and disappearsand therefore, the length of the power transient is proportional to theTC.

An example architecture of a VDC that addresses this issue is shown inFIG. 11. FIG. 11 is a schematic diagram illustrating an example of avoltage-to-digital converter (VDC) that may implement one or moretechniques of this disclosure. As illustrated in FIG. 11, in VDC 1100the two GND PGVs from the M_(n) and M_(n+1) metal layers drive theeven-numbered current starved inverters 1104 a-1104 b and 1106 a-1106 bwithin two identical delay chains, one shown along the top of the figureand one along the bottom. A rising transition is introduced into the topdelay chain ahead of the rising transition introduced into the lowerdelay chain. Given that the M_(n+1) voltage is lower than the M_(n)voltage, the top delay chain propagates the edge more slowly, andeventually, the edge propagating along the bottom delay chain passes thetop edge. Similar to VDC 900 in FIG. 9, as the edges propagate, eachrecords a ‘1’ in a latch as long as it precedes in time the edge on theother delay chain. Otherwise a ‘0’ is stored. The duality of the delaychains causes complementary TCs to be stored in the latches 1108 a-1108n, which are subsequently transferred to the scan chain. An example testresult is given in the center right of FIG. 11 which shows thecomplementary TCs that are produced when the bottom edge passes the topedge at the third latch (not shown).

Although the example architecture of VDC 1100 illustrated in FIG. 11 isalmost two times larger in area than the VDC 900 shown in FIG. 9, itprovides a significant advantage. The power transient signature remainsconstant when the bottom edge passes the top edge, so it is difficult orimpossible for an adversary to determine the precise time at which thisoccurred. Although some information is revealed at the end of the powertransient that reflects the additional time taken by the top edge topropagate to the end of its delay chain relative to the bottom edge, itrequires higher signal-to-noise ratios when analyzing the powertransients to correlate it to the actual PGV voltages. Moreover, it ispossible in this architecture to introduce a ‘stop condition’ where thesignal propagation is halted in the top delay chain, effectivelyeliminating this type of information leakage. For example, by gating theV_(DD) inputs on the top delay chain, it is possible to turn theseinputs off at the instant the bottom edge propagates off the end of itsdelay chain. This action halts the propagation along the top scan chain(and the corresponding power transient) and therefore ‘hides’ thedifference in their delays.

Another example architecture of a VDC that addresses the issues of VDC900 is shown in FIG. 12. FIG. 12 is a schematic diagram illustrating anexample of a voltage-to-digital converter (VDC) that may implement oneor more techniques of this disclosure. As illustrated in FIG. 12, theinputs of VDC 1200 are two voltages labeled VoltInUpper and VoltInLowerand two wires e₁ and e₂ that are connected to an Edge Generator 1202.VDC 1200 outputs two 128-bit thermometer codes (TCs) that reflect themagnitude of the sense voltage inputs. As described above, a TC isdefined as a string of ‘0’s (or ‘1’s) followed by a string of ‘1’s (or‘0’s).

As illustrated in FIG. 12, VDC 1200 is composed of two 256-stage delaychains, where the upper delay chain includes inverters 1203 a-1203 n and1204 a-1204 n and the lower delay chain includes inverters 1205 a-1205 nand 1206 a-1206 n. The VoltInLower input connects to 128 NFETs, insertedin series with the odd-numbered inverters in the delay chain.VoltInUpper connects in a similar fashion to the upper delay chain. ThePUF Engine (not shown in FIG. 12) starts the digitization process bydriving a rising edge into Edge Generator 1202. Edge Generator 1202passes e₁ to the corresponding VDC input, but delays e₂ by a Δt(determined by 32-to-1 select MUX in one example). The two edges then‘race’ down the two inverter chains at speeds relative to the magnitudeof the VoltInUpper VoltInLower inputs.

Under the condition that VoltInUpper>VoltInLower, the edge propagatingalong the top delay chain eventually passes the edge on the bottom delaychain. The outputs of the even inverters along both delay chains connectto a set of latches 1208 a-1208 n that record the point at which thisoccurs. As shown in FIG. 12, the TC produced by the latches on the upperchain is a sequence of ‘0’s followed by ‘1’s, while a complementarypattern appears on the latch outputs of the lower chain. A valueproportional to the magnitude of the voltage difference betweenVoltInUpper and VoltInLower can be obtained by counting the number of‘1’ in either of these TCs. In this manner, VDC 1200 represents anexample of a device configured to digitize a PUF.

Above it was stated that leveraging metal resistance variations as thesource of entropy for a PUF should be inherently more stable acrossenvironmental (TV) variations than leveraging transistor-basedvariations because metal resistance scales linearly with temperature andvoltage. The PGVs used in the analysis presented above actually includevariations from both sources. Although the shorting transistors includedin SMC 200 are very large (e.g., 57× minimum size) and therefore exhibitsmaller variations in comparison to minimum-sized transistors, they dointroduce a component of entropy in the PGV analysis. The entropy worksto improve the results, but the gain is reduced, as is shown below,because of the increased sensitivity of transistor-based variations toTV variations (hereafter called TV noise).

In one example transistor variations may be eliminated by dividing thePGV voltages by the shorting current. These values may be referred to aspower grid equivalent resistances and referenced using the term PGERs.In order to get as ‘pure’ a form as possible of the PGERs, the leakagevoltage and leakage current may also be subtracted from the valuesmeasured with the shorting transistors enabled. The expression for PGERis given by Equation 1:

$\begin{matrix}{{PGER} = \frac{{Vshort} - {Vleak}}{{Ishort} - {Ileak}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

It should be noted that the four measurements used to define the PGEReach may add measurement noise, which is separated and distinguished inthis analysis from TV noise through sample averaging. The PGERdifferences (PGERDs) are created by subtracting pairings of PGERs, aswas done for PGVs as described above.

One of the objectives of the analysis was to show that the PGERDs aremore resilient to TV variations than are the PGVDs. In order todetermine the magnitude of the TV variations (or ‘noise’), the PGVD andPGERD data was calibrated. Calibration removes the DC offsets introducedby TV noise in the data, but preserves the variation. Calibration may becarried out by computing the mean PGERD and PGVD over the entire set ofSMCs for a given metal layer pairing and TV corner. Correction factorsmay then be computed by subtracting the mean value at each of the TVcorners from a reference TV corner. The reference is the data collectedat 25° C., 1.2V. The correction factors are then added to thecorresponding data from the TV corners.

A subset of the calibrated M₂-M₃ PGERDs and PGVDs computed using datafrom one of the chips is shown in the graphs illustrated in FIG. 13A andFIG. 13B, respectively. In FIGS. 13A-13B the SMC number is given alongthe x-axis and the PGERD or PGVD calibrated to 25° C. is plotted alongthe y-axis. Each point in the curves is the average of 11 samples (thesamples are also plotted as unconnected points to illustrate measurementnoise). The averaging eliminates most of the measurement noise.Therefore, variations in the line-connected curves are introducedprimarily by TV noise.

The 3σ values listed in the PGERD plot indicate that TV noise is approx.2.7 times the measurement noise (1.665/0.620). Bitflips occur when theslopes of the lines between any two adjacent pairing of points reversessign (examples are shown in FIG. 13A). In contrast, the ratio increasesto 7.7 (23/3) for the PGVD analysis. Therefore, TV noise is nearly threetimes more likely to introduce a bitflip in the PGVD analysis than inthe PGERD analysis.

As described above, Equation 1 requires measuring current values.Measuring current on chip may be difficult. FIG. 14 illustrates aschematic diagram of an example measurement circuit that may be used asan alternative to measuring current values on chip. In one example,circuit 1400 may normalizes voltage drops for SMCs within the block to areference current. As illustrated in FIG. 14, circuit 1400 includes ANDgate 1402, pass gate 1404, pass gate 1406, NFET 1408, resistor 1410, andresistor 1412, which represents the entropy source. AND gate 1402 may beconfigured to enable a particular SMC. Pass gate 1404 and pass gate 1406may be connected across an entropy source. For example, pass gate 1404and 1406 may be respectively connected to PS and NS. NFET 1408 may beconfigured to provide stimulus when an SMC is enabled. In one example,NFET 1408 may be configured to provide 500 μA when SMC is enabled.Resistor 1410 may be a controlled-resistance silicide poly resistor. Inone example, resistor value 1410 may have a resistance value ofapproximately 400 Ohms. In this manner, when an SMC is enabled, the NFET1408 current creates a voltage drop across the entropy source which canbe sensed by pass gate 1404 and pass gate 1406. Circuit 1400 may allowvoltage variations introduced by the different NFET currents within theSMCs of the block to be eliminated. The process, may be referred to ascalled normalization and is described below.

The objective of normalization may be to eliminate transistor currentvariations as a component of the measured voltage drops across theentropy stack. Previous work suggests that the current-inducedvariations contribute significantly to TV noise, which, in turn, acts toreduce the probability of correctly regenerating the bitstring.Normalization may be thought of as a process that ‘normalizes’ thevoltage drops for all SMCs within the block to a reference current.Normalization is derived from the basic circuit theory equation R=V/Igiven by Equation 2 below which states that the resistance of theentropy source can be obtained from the sense voltage measurements bydividing through by the NFET current. Unfortunately, measuring currentson-chip is challenging and impractical.

$\begin{matrix}{R = \frac{\left( {{VsenseUpper} - {VsenseLower}} \right)}{INFET}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Equation 3 provides an alternative in cases where it is only necessaryto determine a value that is ‘proportional’ to resistance. Here,DVSenseUpper is the digitized voltage (a value between 0 and 128 fromthe VDC) that is produced at the higher voltage point across the entropysource as shown in FIG. 14. Current from the enabledstimulus-measure-circuit (SMC) in the block flows across theR_(parasitic), (1410). Therefore, the voltage drop defined by(129−DVSenseUpper) is proportional to the NFET current.

$\begin{matrix}{R = \frac{\left( {{DVsenseUpper} - {DVsenseLower}} \right) \times 256}{\left( {129 - {DVsenseUpper}} \right)}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

In this manner, the circuits described herein represent example circuitsconfigured to generate physical unclonable functions. Various exampleshave been described. These and other examples are within the scope ofthe following claims.

What is claimed is:
 1. A device for generating a digital value for aphysically unclonable function, the device comprising: a first voltageinput, a second voltage input, an edge generator, wherein both the firstvoltage input and the second voltage input are connected to the edgegenerator, an upper delay chain including one or more odd-numbered upperinverters and one or more even-numbered upper inverters, a lower delaychain including one or more odd-numbered lower inverters and one or moreeven-numbered lower inverters, one or more field effect transistors,wherein the first voltage input connects to the upper delay chainthrough the one or more field effect transistors and the second voltageinput connects to the lower delay chain through the one or more filedeffect transistors, a first output of a first thermometer code from theupper delay chain and a second output of a second thermometer code fromthe lower delay chain, wherein a value proportional to a voltagedifference between the first voltage input and the second voltage inputis obtained based on one or more of the first thermometer code and thesecond thermometer code.
 2. The device for generating a digital valuefor a physically unclonable function according to claim 1, wherein theone or more field effect transistors are inserted in series with theodd-numbered upper inverters and the odd-numbered lower inverters. 3.The device for generating a digital value for a physically unclonablefunction according to claim 1, wherein the field effect transistor is aNFET.
 4. The device for generating a digital value for a physicallyunclonable function according to claim 1, wherein the thermometer codeis a 128-bit thermometer code.
 5. The device for generating a digitalvalue for a physically unclonable function according to claim 1, whereinthe first thermometer code is a string of ‘0’s followed by a string ofor a string of followed by a string of ‘0’s.
 6. The device forgenerating a digital value for a physically unclonable functionaccording to claim 1 further comprising one or more sets of latches. 7.The device for generating a digital value for a physically unclonablefunction according to claim 6, wherein the first output from the upperdelay chain and the second output from the lower delay chain eachconnect to the one or more sets of latches.
 8. A method for generating adigital value for a physically unclonable function comprising the stepsof: connecting an edge generator to both a first voltage input and asecond voltage input, providing an upper delay chain and a lower delaychain, each delay chain including one or more odd-numbered inverters andone or more even-numbered lower, connecting through one or more fieldeffect transistors the first voltage input to the upper delay chain andthe second voltage input to the lower delay chain, generating a firstthermometer code from the upper delay chain and a second thermometercode from the lower delay chain, passing by the edge generator a firstedge along the upper delay chain and a second edge along the lower delaychain, recording by one or more latches a point at which one of thefirst edge and the second edge passes the other edge along the delaychain, producing by each of the one or more latches a thermometer codefor each of the upper delay chain and the lower delay chain, obtaining avalue representing a voltage difference between the first voltage inputand the second voltage input based on the thermometer code.
 9. Themethod for generating a digital value for a physically unclonablefunction according to claim 8, wherein the producing step furthercomprises the steps of: generating a string of ‘0’s followed by a stringof or a string of followed by a string of ‘0’s, and counting a number oftimes appears in each string.
 10. The method for generating a digitalvalue for a physically unclonable function according to claim 8 furthercomprising the step of eliminating transistor variations by dividingpower grid voltages by shorting current.